Use kPointerSize to compute the size of StubCache::Entry for X64

R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/17552004

Patch from Haitao Feng <haitao.feng@intel.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15272 ce2b1a6d-e550-0410-aec6-3dcde31c8c00